The simple fact the online rebooking tool is flaky, a mess, a disaster, .... it has been a "problem" for years and best avoided (as suggested in the wiki).
One should always use the regular search tool first, find the desired flight
Then use the re-booking tool, if the desired flight appears or is priced correctly (a re-occurring problem), fine --- but be prepared to be disappointed on anything but the simplest of changes
Call and feed the phone agent the desired flights and you should be done in a few minutes, with the right pricing